LEAF PHOTOSYNTHESIS, STOMATAL-RESISTANCE, AND GROWTH OF WINE GRAPES (VITIS-VINIFERA L.) AFTER EXPOSURE TO SIMULATED CHLORSULFURON DRIFT

The lower Yakima Valley of Washington is a highly diversified irrigate d agricultural region bordered by dryland wheat fields. Residues of he rbicides sprayed in wheat can be atmospherically transported to suscep tible nontarget crops in the valley. Nontarget crops may be exposed re peatedly to low levels of herbicide residues. The effects on grapes ex posed to phenoxyacetate herbicide drift has been well documented, but the effects of comparatively newer wheat herbicides like sulfonylureas (SUs) are less known. Potential effects of repeated exposures of grap es to an SU herbicide were assessed in a simulated drift study. Grape vines of the cultivar ''Lemberger'' were sprayed up to three times at a weekly interval with 1/100 (0.01X) of a field application rate of ch lorsulfuron, which is a postemergence wheat herbicide. Thirty-five day s after the first application, photosynthesis and stomatal resistance of randomly tagged, fully expanded leaves were measured. Total leaf ar ea and chlorotic leaf area were also measured on the same leaves. The percentage of chlorotic area was calculated to determine the correlati on between chlorotic area and photosynthetic activity of the leaf. Fol iar injury was indexed on a numerical scale and scored in three interv als over 120 days of growth. The severity of leaf injury increased aft er multiple exposures to chlorsulfuron. but symptoms had dissipated by 120 days after application. Leaf photosynthesis was reduced and stoma tal resistance was increased linearly with an increase in the percenta ge of chlorotic leaf area. Three consecutive applications of chlorsulf uron resulted in 50% chlorotic leaf area that was associated with 25% reduction in the net leaf photosynthesis and a 59% increase in the sto matal resistance, The reduction in photosynthesis was probably caused in part by an increase in stomatal resistance and in part by a large i ncrease in the chlorotic leaf area. Shoot length and pruning weight we re not affected by the number of exposures. However, leaf area was sig nificantly reduced following three weekly exposures. The results sugge sted that significant reduction in photosynthetic activity only occurr ed when general leaf chlorosis exceeded 50%. Isolated diffuse chloroti c spots have previously been reported as characteristic of nonpoint at mospheric deposition of SU herbicide residues. However, our results sh owed that such symptoms should not adversely affect plant photosynthes is.